1. Field of the Invention
This invention relates to a magneto-optical information recording apparatus, and more particularly to a magneto-optical information recording apparatus which can produce a magnetic field on the surface of a disk-like magneto-optical recording medium in a predetermined direction and can record, reproduce or erase information with non-contact by application of a light beam.
2. Related Background Art
Magneto-optical disk apparatuses using, for example, disk-like recording media are known as magneto-optical information recording apparatuses.
As the magneto-optical recording medium used in such apparatuses, use is made of a medium in which a vertically magnetized film is formed on the surface of a disk and the directions of magnetization of the vertically magnetized film are arranged in a predetermined direction.
Recording of information is accomplished in such a manner that a laser-modulated laser beam is applied to the vertically magnetized film and the temperature of the vertically magnetized film exceeds the curie point (160.degree.). The portion to which the laser beam has been applied at this time is made opposite in direction of magnetization to the surroundings thereof by a DC bias magnetic field, and is again magnetized. In this manner, a pit row comprising a magnetization-inverted portion is formed in conformity with an information signal.
Also, to read the information recorded on a magneto-optical disk, there is a reading method utilizing the so-called magnetic Kerr effect which comprises applying a reading laser beam to the vertically magnetized film and changing the direction of the reflected beam by the difference in the direction of magnetization of the vertically magnetized film.
To erase the recording pits, a laser beam is applied to the recording pit portion while tracking on the recording pits, and a DC bias magnetic field in the direction of magnetization opposite to that of the recording pits is applied, and the directions of magnetization of the vertically magnetized film are again made uniform.
Here, it is to be understood that the DC bias magnetic field is changed over in direction of magnetization during the recording and during the erasing.
As a method of producing a DC bias magnetic field, there is generally a method using a permanent magnet or a method using an electromagnet.
However, according to the former method using a permanent magnet, it takes much time to invert the magnetization and also, a mechanism for inverting the magnetization is required, and this leads to an increased number of parts and thus, an increased cost. For this reason, the latter method using an electromagnet has heretofore been adopted as the method of producing a DC bias magnetic field.
FIG. 1 of the accompanying drawings is a side cross-sectional view schematically showing the construction of a magneto-optical disk apparatus according to the prior art for recording information on a magneto-optical disk.
In FIG. 1, reference numeral 100 designates an optical disk. A plurality of row-like tracks comprising concavo-convexities are formed on the optical disk 100. A substrate 102 is provided on the upper surface of a vertically magnetized film 101 and a lining plate 103 is provided on the underside of the vertically magnetized film 101, whereby the optical disk 100 is constituted.
TbFe or TbFeCo is known as the material forming the vertically magnetized film 101, and glass or plastics is known as the material forming the lining plate 103.
An objective 104 is disposed for movement in the radial direction of the optical disk 100 with an optical head, and is finely driven in the direction of the optic axis and the radial direction of the disk by objective driving means 105 provided in the optical head, whereby focusing or tracking control is accomplished.
The objective driving means 105 is a device which drives the objective 104 by an electromagnetic force. Although not shown, this objective driving means comprises, for example, a coil and a magnet producing a magnetic field across the coil, and one of these is fixed to the objective and the other is fixed to the optical head body, and the objective driving means 105 moves the objective 104 by a drive force produced by flowing an electric current to the coil.
The objective driving means 105 and the objective 104 are contained in a protective outer frame 107 provided with a light-passing opening 106 at the bottom thereof. The protective outer frame 107 is usually formed of plastics or the like and prevents extraneous dust or the like from adhering to the surface of the lens.
A yoke 108 comprises, for example, an E-shaped magnetic member, and forms a bias magnet by an electric current flowing in a coil 109 wound on the protruding portion l08a of the yoke 108. By this bias magnet, an upward magnetic field is applied to the vertically magnetized film 101. The magnetic line of force having left the central protruding portion of the yoke enters the opposite protruding portions of the yoke via the routes as indicated by broken lines in FIG. 1. Accordingly, the magnetic flux density distribution in the vertically magnetized film 101 widens in the direction of the film surface and part of the magnetic flux provides a leakage flux which does not contribute to recording. In such a construction, to obtain the intensity of magnetic field necessary for recording, it becomes necessary for a great electric current to flow in the coil 109 or to increase the number of turns of the coil 109.
However, if a great electric current flows in the coil, a bulky power source of great capacity will become necessary and temperature rise will also become large. Also, where the number of turns of the coil is increased, the bias magnetic will unavoidably become bulky and the reactance of the coil will increase with the increase in the number of turns and, in such case, much time will be required for recording, erasing or converse change-over.